Selenite is a mostly clear, transparent variety of the sulfate mineral gypsum. The name selenite is also commonly used for other varieties of gypsum, including satin spar gypsum, desert roses, and gypsum flowers. All varieties of gypsum, including selenite, satin spar, and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no selenium; the similar names both derive from Greek selḗnē (σελήνη 'Moon'). Some of the largest crystals ever found are of selenite, the largest specimen found in the Naica Mine's Cave of the Crystals being 12 meters long and weighing 12 tons.

Closest relatives